Liquid casting aluminum is a single phase alloy material to which other elements are added to aluminum to form a solution, i.e., are dissolved in the aluminum. When the casting aluminum alloy is allowed to slowly cool from a melt phase, e.g., standing in open air, the added elements in the aluminum precipitate out of the solution through a process known as nucleation. Nucleation in material allowed to slowly cool is a process in which not many nuclei are formed, but the ones that do form grow rapidly in size and consume the added elements. The result is a bulk article wherein the aluminum is relatively pure metal with isolated, distinct volumes of added elements distributed non-uniformly through the aluminum. This state is undesirable when the aluminum is being used to form structural articles because pure aluminum is gummy to machine and has poor strength properties.
But this is the current state of practice and has been since the development of aluminum casting processes approximately 120 years ago. Typical today is a process whereby a significant number of castings are formed and allowed to cool on the shop floor (losing the heat of the melt to the atmosphere) until a significant quantity are collected and the batches are heated in a convection oven to the “solutionizing” temperature (approximately 1,000° F.) which in theory allows the large nuclei to breakdown and migrate as smaller particles distributed uniformly throughout the parent material. After a sufficient time for migration (typically 2-8 hours at temperature) the castings are dropped into the quench tank and rapidly cooled, locking the added elements in a uniform distribution throughout the parent (e.g., aluminum) material. The goal of this process is to return the precipitate in the solid material to near the solution state of the liquid material that was poured into the mold. This cooling and reheating, given the efficiencies of the convection oven, has an energy cost at a minimum of four times the energy required to form the original melt.
There are many issues with current heat treatment processes even though they are nearly universal in application. The current processes are not accurate with regard to the uniformity of temperature of the batch heating process. Additionally, as the cold castings are typically stacked one upon another prior to submission to the heat treatment oven, there is the potential for damage, and as the large stack of hot castings is inserted into the quench tank, the temperature of the quench tank changes significantly from the first casting inserted to the last one. The existing processes are expensive in terms of both time and energy, in that the heat treatment cost is a major component of the total production cost.
Typically the quenched casting is allowed to age naturally or it is submitted to an artificial aging process where the castings are inserted in a fixed temperature convection heating oven for a period of time. Artificial aging is desirable because it saves time and allows the parts to move to machining sooner. But the convection heating of the cold casting to the artificial aging temperature requires significant additional energy and time.
The current process including casting, heat treatment and aging process can take a minimum of a week and in most foundries a typical process flow will require more than three weeks to move from pour through machining with most of those three weeks involving heat treatment and artificial aging.
Most significantly, when this inventor began this study and research effort nearly 18 years ago, there was little interest in the energy lost to the conventional heat treatment, quench and artificial aging processes. But with current sensitivities to energy waste the opportunity to save as much as 90% of the energy of the process is now of keen interest.
If aluminum with added elements is rapidly cooled (quenched) while still hot and in a solutionized state, the added elements do not have the opportunity to form large nuclei, but will form many uniformly distributed smaller nuclei as the metals solidify. If the aluminum and added elements have been allowed to slowly cool to a solid, it is still possible to add enough thermal energy to force the added elements to dissolve in the aluminum (solutionize) without actually melting the aluminum. The advantage this offers is that the aluminum can be cast into shape, cooled in air until sometime later, and then re-solutionized and quenched. The more uniform distribution of many nuclei dramatically increases the strength of the alloy over pure aluminum or alloys that cool too slowly and form far fewer and much larger nuclei.
The aluminum alloy casting can be developed to further improve its mechanical strengths by growing the size of the many uniformly distributed nuclei through a process of precipitation heat treating or precipitation hardening typically referred to as “aging.” During “aging,” the nuclei grow larger in size as a diffusion process that progresses more rapidly at elevated temperatures. But, if the temperatures are elevated too high, upon cooling the nuclei will collapse together and form fewer large nuclei similar to that found in aluminum that has been slowly cooled, e.g., without solutionizing, as described above. Ideally, precipitation strengthening of supersaturated solid solutions involves the formation of finely dispersed precipitates during aging heat treatment (which may include either natural aging or artificial aging). The aging must be accomplished not only below the equilibrium solvus temperature, but below a metastable miscibility gap called the Guinier-Preston (GP) zone solvus line. The GP zone solvus line is the temperature for a particular mix of parent metal and added elements above which the magnitude of super-saturation decreases the probability of stable clusters being formed. The eutectic mix is not in equilibrium, but at elevated temperatures above the GP zone solvus line, the extremely fine-grain molecular foundations upon which the precipitates will grow and harden the physical properties, become unstable and do not support precipitation growth or, at higher temperatures, even dissolve.
The aluminum casting industry uses heat treating as a mechanism to increase the strength of the aluminum castings. The process usually amounts to the addition of sufficient thermal energy to force all of the elements that have been added to the aluminum into a solid solution (solutionizing). Energy is consumed as these added elements are dissolved in the aluminum solid. The amount of energy required to achieve the necessary diffusion is significant.
Conventional methods for producing cast aluminum alloy products include initially pouring a suitable molten aluminum alloy into a mold. After the molten alloy has sufficiently solidified, the casting is removed, and is set aside to cool in the open air. Normally, a few days' worth of production is collected for a batch solutionizing process. Alternatively, the removed casting could be immediately subjected to a solution heat treatment without cooling first.
A conventional method for solution heat treating a cast part involves placing many cast parts in a large forced air convection oven. In the convection oven, the castings are subjected to circulated air or some non-reactive gas which has been heated to the desired “solution” temperature (approximately 1,000° F.). Typically, the process is procedural and the castings are held for what is deemed a sufficient amount of time for them to “process”, usually at least 2-8 hours. Following the solution heating phase, the cast part is immediately quenched in water to rapidly cool the product. Following this cooling, the part is naturally or artificially aged.
One of the drawbacks of the convection solution heat treatment processes, such as that described above, is the length of time required to complete the treatment. Typically, large numbers of cast aluminum parts are solution heat treated at once in a batch process. Since it is difficult to maintain even and uniform temperatures in all of the parts, in order to ensure that all the parts are properly heated, the exposure time to the hot convection gases is usually at least two hours and often more than eight hours. The length of time required for the solution heat treatment contributes significantly to the time required to manufacture cast parts. Furthermore the stacking of the parts may seriously interfere with the ideal convection air flow resulting in a non-uniform heating profile between parts heated in the same batch.
It has been proposed that infrared heat treatment systems may improve the operational efficiency of the casting process by replacing convection air driven solution heat system and reducing cycle times. For example, U.S. Pat. No. 5,306,359 describes a method for heat treating an aluminum part by applying infrared radiation directly from a source of infrared energy to the part until the part attains a desired state of heating. According to the '359 patent, during the heat treating, the temperature of the part is monitored and the intensity of the radiation source is proportionally controlled in response to the monitored temperature. The temperature of the part in the '359 patent is described as being monitored by a plurality of optical pyrometers 80, 82 and 89, illustrated as being directed towards an irradiated surface of the part.
The '359 patent stated that the use of optical pyrometers to measure the temperature of the aluminum cast parts is complicated by the reflectivity of aluminum and the uncontrolled radiant energy from the background (i.e., the temperature of the lamps, and refractive surfaces). Reportedly, the reflectivity of the aluminum and the radiant energy of the background cooperate to create a temperature readout from the optical pyrometers that is not representative of the temperature of the surface of the part being observed by the optical pyrometers. In an effort to account for these inaccuracies and provide a more accurate reading of the temperature of the part, the '359 patent describes the taking of measurements from a background optical pyrometer, then making adjustments to the readout from the part optical pyrometer based on the readout from the background optical pyrometer.
U.S. Pat. No. 5,336,344 describes a method and apparatus for producing a cast aluminum part using a high intensity electric infrared heating system to heat the part. The described system is similar to the system described in U.S. Pat. No. 5,306,359 noted above. The '344 patent broadly describes that each infrared heating station includes a means for monitoring the actual temperature of the casting, and that the heating of the casting at each station is controlled in accordance with this monitored temperature. Like the '359 patent, the '344 patent describes that optical pyrometers 46 can be used to generate a signal representative of the casting temperature. The '344 patent describes that this signal can be used to control the heating of the parts. In the illustrations, the optical pyrometers are shown as being directed at a surface of the part that is irradiated.
U.S. Pat. No. 5,340,418, by the same inventor of the '344 patent, proposes additional control methods to control the amount and application rate of infrared energy applied to the part during the solution heat treating process. These proposed methods rely upon the same optical pyrometers described in the '344 patent for assessing the part temperature. In one embodiment, the optical pyrometers are used to monitor the temperature of the part. This temperature is compared to a predetermined solution heat treatment temperature which is chosen as a function of the particular material used to cast the part. As long as the temperature of the casting as measured is less than the predetermined solution heat treatment temperature, the heating is continued at the initial predetermined level provided by the infrared energy source.
In each of the processes described in the three patents noted above, the cast aluminum part is indexed through a plurality of individual stations while the part is rotated relative to the path of travel. By indexing the part through the stations, the part resides in each station for a predetermined period of time before it is transported to the next station.
Industry expectations for each of the processes and apparatuses described in the patents noted above were high; however, practical experience has shown that the processes and apparatuses described in the above patents have not found commercial acceptance due to difficulties in producing cast aluminum parts with reliable physical properties, such as strength. Accordingly, there continues to be a need for improvements to processes for solution heat treating cast metal alloy parts using infrared energy as a heat source.
This inventor has spent more than 10,000 (independent, not for hire) hours over the last 15 years consulting with experts in the various fields of physics, metallurgy, infrared emitters and refractory technology. This research activity resulted in several different renditions of an infrared heat treatment apparatus and an evolved method of application. As part of this research and development effort several specific issues defining the previously cited systems and many of the processing systems operating in some of the largest casting facilities in the USA were collected and are included as background to the presented apparatus and method.
Specific issues were noted upon examination of several infrared systems (e.g., heat treatment, coating curing, industrial and chemical processing) corrections for which became components of the design rules incorporated into the present embodiment:
Inefficiencies of T-3 Bulb as a Radiating Emitter
Many systems use the T-3 quartz incandescent high-output lamp with a radiant source temperature of 2,204° C. In fact, the tungsten element can reach temperatures exceeding 3,000° C. But the quartz bulb cannot. Quartz softens near 1,660° C. More importantly, the Quartz bulb is at best less than 90% transmissive at wavelengths of greater than 150 nm. The quartz bulb is only about 90% transmissive at 1.25 μm, which is approximately the center point of the emitted wavelengths for a system with a characteristic temperature of 2,204° C.
The quartz bulb of a 1 kW emitter system must be cooled at a rate that will limit the temperature of the bulb to prevent it from melting. This will require removing at least 100 Watts of thermal energy while the balance of the energy is radiated in the pattern of roughly a cylinder as a Lambertian radiator. A Lambertian radiator will appear to have the same radiant power at any angle of observation. This means that the tube of the T-3 lamp will emit maximum power normal to its long axis in a full 360° radiating pattern and will appear to an optical sensor to be of constant brightness at any viewing angle.
Attempts at placing reflectors inside the heat treatment oven are problematic. Ideally a full parabolic trough reflector should surround each T-3 bulb. To be effective the reflector would redirect nearly 270° of the full radiating pattern towards the intended target. Such a reflector would also direct a significant portion of the radiated power back through the bulb, causing a significant increase in bulb heating. The result is that less than a quarter of the radiant energy is radiated at the temperature of the tungsten element and passes through the quartz bulb towards the intended target, while nearly three quarters of the radiant energy is radiated off axis most of which is intended to be captured by the reflector system and directed towards the target. As such, the effective radiated power to reach the target is dramatically reduced. The lost energy must be collected by the bulb cooling system. Bulb cooling systems usually involve water cooled or air-cooled trough reflectors and bulb end caps.
A further revelation relates to the water-cooled bulb end-caps required to cool the quartz bulb. U.S. Pat. No. 8,865,058 locates the bulb caps outside of the oven proper and flows cooling gas around the bulbs to keep the quartz from getting soft. Water-cooled bulb caps and water-cooled reflectors are commonly located inside the oven proper on many existing infrared heat treatment or coating curing ovens. Either approach presents a serious radiant energy management problem inside the oven proper that will be examined later in this disclosure.
Appropriate Wavelengths to Use When Heating Aluminum
The T-3 radiant system is usually chosen because of its high power output, however the high temperature of the T-3 may not be the optimum radiant energy source for the application of heat treating aluminum castings. The question is, “Does the shorter wavelength with the advantage of the more effective radiant energy transmission outweigh the longer wavelength and the potential for increased absorption?”
From Einstein's writings about coefficients for emission and absorption (a and b coefficients discussed in Einstein, A. (1916), “Strahlungs-Emission und-Absorption nach der Quantentheorie”) it can be deduced that good reflectors will also be good absorbers. From the Planck Hypothesis, high frequencies (shorter wavelengths) have greater energies and also from the Planck Hypothesis electromagnetic energies only exist as discrete quanta or photons. Of course these works are foundational to Laser theory, but in the early 1900's these works had thermodynamics as their field of focus. Here they point to the fact that absorption and reflection involve additional thermodynamic considerations that are wavelength dependent. For some metals, shorter wavelengths below and through the visible spectrum involve photon energies that match available electron band energy levels; but other materials, typically non-conducting materials, are transparent to high-frequency electromagnetic radiation and opaque to lower frequencies. Care must be taken here because what appears to be “transparent” can actually be an absorption and a retransmission within the material. The metallic radio antennas on a cell phone do not reflect electromagnetic energy but they are very good at absorbing it and retransmitting it within the material, i.e., conducting electricity along its length. This is true when the antenna is tuned or impedance matched, a term that relates the wavelength of the electromagnetic radiation and the speed of propagation or the effective transmission within the metal conductor which, done perfectly, eliminates any reflection.
Planck and Einstein provided some evidence that radiating energies beginning from a solid body (ideally a blackbody) are released in discrete quantities and these energies are captured when striking a solid body (ideally a blackbody) in discrete quanta. We get some idea of these issues when we heat most metals that are a solid at room temperature and the human eye sees the material radiate red. The human eye has a limited bandwidth, termed the “visible spectrum.” Thermal spectroscopy is the science of recognizing the elements by their wavelength of emission over a relatively wide spectrum (wider than the “visible spectrum”) as the material heats and its temperature rises.
The properties of aluminum and aluminum castings are one of the principal focuses of this disclosure; as such, it is important to note that unlike materials from copper to iron, aluminum does not turn red before it melts. Thus this inventor has made some assessment of this fact to drive the development of a quite hot infrared emitter that can be tuned to wavelengths longer (i.e., lower temperature) than the typical operational wavelength of the T-3 emitter. The emitter disclosed in this patent has a minimum wavelength approximately 50% greater than the characteristic wavelength of a high-power incandescent bulb such as the T-3 system. But more importantly, the new emitter can be tuned to effectively radiate at wavelengths much longer than competing technologies. Still, there are compounding considerations such as the surface texture of the casting. These physical aberrations can be viewed as somewhat akin to the properties of an antenna and treated with some aspects of antenna theory.
Appropriate Wavelengths are Relative to Surface Roughness
Reflectance is inversely proportional to surface roughness and directly proportional to wavelength. Polished surfaces of high-conductivity metals with an abundance of free electrons make good reflectors. These considerations are compounded by the non-polished surfaces of the aluminum casting removed directly from the mold. Here, if the wavelengths are short compared to the surface roughness features and yet long enough to be more likely to cause thermodynamic absorption and transmission within the material, then a very high degree of the radiant energy will be coupled into the casting (C.D. Wen International Journal of Heat and Mass Transfer 49 (2006) 4279-4289). Again this has an analog to electromagnetic radiation and some aspects of antenna theory. The fundamentals of quantum theory and thermodynamics were the basis for some of Einstein's early work. Although it is sometimes difficult to understand, this inventor has tried to relate an understanding of this analysis to the observed facts that have been collected over nearly 20 years of field and laboratory research.
Typical aluminum castings from permanent molds have surface roughness rates on an Ra scale (average peak to valley over a unit area of the surface in micrometers) of about 2 μm to about 3.3 μm. This compares to the Wien's Displacement Law for Blackbody Radiator's relating the stated wavelengths to temperatures from about 600° C. (1,100° F.) to about 1,200° C. (2,200° F.).
From the above discussion, this inventor recognized that aluminum has much higher thermal conductivity than iron, but significantly lower than copper. Yet both copper and iron will glow red before they melt. Aluminum does not. Aluminum appears to have a better thermodynamic “impedance match” to infrared radiant energy than most other metals and as such is a better absorber of thermal radiation of the optimum wavelength.
This means that for a given roughness of the aluminum surface there is an optimum wavelength to minimize reflected radiant energy and maximize the absorbed radiant energy. The optimum radiant source (e.g., infrared emitter) for heat treating aluminum castings will be one that can be tuned over the wavelengths of interest (i.e., from about 2 μm to about 3.3 μm). Such a method for the estimation of an optimum wavelength given a measured surface roughness (e.g., using optical non-contact surface profilometer) and a process of creating the optimal thermal profile would include driving the radiant sources to the optimal temperature (i.e., wavelength) for the necessary time of exposure.
The Misconceptions about Reflectivity in a Stefan-Boltzmann Environment
All of the previous oven systems examined by this inventor present a similar problem of radiant energy management inside the oven. There is much discussion about the reflectivity of the materials inside the oven proper. In fact, the nature of the oven interior is subject to the Stefan-Boltzmann Law for radiant energy sources. As derived from the Stefan-Boltzmann Law, radiant energy flows from the hotter source to the colder receiver as the fourth power of the difference in the temperatures between the source and the receiver. The cooled end caps and or the cooled reflectors inside the oven proper of the T-3 bulb systems become extremely effective infrared energy sinks and will consume much of the available radiant energy inside the oven proper, especially as the temperature of the casting increases to be near the solutionizing temperature. Cooling the T-3 bulbs with forced gas flow inside the oven reduces the energy available to heat the casting. The end result is significantly lower radiant energy transfer efficiencies than anticipated by the systems designers.
Optical Infrared Sensors in a Stefan-Boltzmann Environment
All of the above considerations are compounded by the incorrect assumptions made relative to the use of a radiant energy sensor (the pyrometer) in the presence of the high output energy sources as compared to the energy emitted from the casting. As shown in the Stefan-Boltzmann Law, the effectiveness of the radiant energy transfer is proportional to the 4th power of the difference in temperature. Using a properly cooled infrared sensor to examine the casting at a temperature of approximately one quarter of the temperature of the infrared source would indicate that the high temperature source would dominate the radiant energy of the casting by a factor of more than 200 to one (given the Stefan-Boltzmann Law). There is nothing that could be gained by measuring the background temperature and trying to compute the actual temperature of the casting in the presence of the high temperature radiant source given the exponential relationship.
Considerations for Shielding Optical Window
The field surveys by this inventor also found that all of the fielded systems examined did not properly shield the infrared sensor or pyrometer from continuous exposure to the radiant energy in the solutionizing oven. Prolonged exposure of just a few seconds will cause the “window” of the sensor to heat up and become the dominating radiator in the sensor's field of view. Even actively cooling the housing of the sensor window will not effectively eliminate the thermal contamination of the measurement.